Narrator: We have a mysterious new kid on the block. Astronomers call it Oumuamua. Which in Hawaiian, roughly translates to “first time visitor from far away.” Oumuamua is the first interstellar object ever detected in our solar system. Astronomers discovered it in 2017 with the Pan-STARRS1 telescope. And like all grand discoveries, it has raised more questions than it answers. Where did it come from? What’s it made of? And yes, even the ultimate query, is it aliens? That’s what two Harvard astrophysicists posed in a recent paper, suggesting that Oumuamua could be some type of alien spacecraft. And when we spoke with one of them, the idea didn’t sound as far-fetched as you might think.

Loeb: I prefer to adopt the maxim of Sherlock Holmes that if you rule out the impossible, whatever remains, as improbable as it is, must be the truth.

Narrator: Now a good rule of thumb is that aliens are always the least likely answer. But here’s the thing, the more reasonable explanation right now isn’t panning out. Most astronomers think that Oumuamua is probably a stray comet from another stellar system. The only problem is that it doesn’t look like any comet we’ve ever seen, because it appears to lack that iconic tail.

Loeb: There was an anomaly seen in the data. And it cannot be explained by the most conventional explanation, which in this case is a cometary outgassing. And so we propose an alternative interpretation which is the only one I can think of. Narrator: But it seems extreme to assume aliens just because Oumuamua doesn’t look like a comet, right? Not when the Harvard scientists accounted for another peculiarity. As Oumuamua traveled through our solar system, it didn’t follow the normal path of a typical comet under the sun’s gravity. Rather, it slightly shifted off course, which couldn’t be explained by gravity alone. Something else, some unknown force was also at play, manipulating the object’s behavior.

Loeb: I cannot think of any other possibility other than the outgassing that we find in comets, which seems to be ruled out because we don’t see a cometary tail, or the pressure from the sunlight. There is no other proposal on the table right now.

Narrator: Pressure from sunlight. It works similarly to how wind pushes against a sail on a boat. So too can sunlight push against a spacecraft to propel it through space. Incidentally, we call these spacecraft solar sails. There’s just one catch. Pressure from sunlight is extremely light. Lighter than the weight of a fly on your hand. Which means in order for it to be effective, solar sails also have to be extremely light and thin. In 2010, for example, Japan’s space agency launched its Ikaros solar sail, which only weighed 1.1 pounds. But it eventually managed to travel over 65 million miles from Earth on just sunlight alone. And it’s this type of force that the Harvard scientists think could be propelling Oumuamua too. Of course, that’s if it’s an alien made sail in the first place.

Loeb: Well first of all, I am not confident that this is the correct interpretation because we don’t have enough data. And so this is one possibility. I would highly recommend people to read the paper. If they have a better idea, they should publish it.

Narrator: And as always, not everyone agrees with Loeb’s hypothesis.

Weryk: I think that their explanation, I think it’s in contrast with what the data says. You know, we did consider that possibility in our own paper, which was published earlier, and we dismissed it as not being physically valid. You know again, there’s no reason to think Oumuamua is anything but a natural object.

Narrator: That’s Rob Weryk, who first discovered Oumuamua in 2017.

Weryk: So we think Oumuamua still has ice and the sublimating ice gives it a small tiny kick that gravity alone wouldn’t account for, but that the dust it has is much larger than what comets typically have. And so we just don’t see that from the ground.

Narrator: And yes, in all probability, he’s right. Oumuamua is a natural object, like a comet or asteroid. But neither Weryk nor Loeb can know for sure without more data. And sadly, Oumuamua is on its way out of the solar system beyond the reach of our telescopes. So the best bet for solving this mystery?

Weryk: So for Oumuamua itself, it’s basically gone, it’s too faint to see from the ground. There’s no way we could send a spacecraft mission, so we really have to find a second object. That would help answer a lot of questions that we don’t know. You know, I’m very interested in finding a second one. It’d be nice if I found them all.

Narrator: And Loeb already has some ideas on where to look next.

Loeb: There should be quite a lot of them right now in the solar system. And some of them could be trapped by Jupiter and the sun that act as a fishing net. And so some of these interstellar objects are bound to this solar system after the first passage.

Narrator: That’s right. Some of these interstellar objects might be right here in our solar system, trapped, and waiting to be found. It looks like astronomers may have more fascinating discoveries to come.